The cursor circuit invention disclosed herein is employed as a subsystem in the video generator circuit for a dynamic digital television display disclosed in U.S. Pat. No. 3,996,585, assigned to the instant assignee, which is a continuation-in-part of Ser. No. 478,816, by W. J. Hogan and A. A. Schwartz, filed June 11, 1974 now abandoned. This video generator circuit system converts randomly occuring data signals representing graphical patterns into a time sequential video signal for use with a sequentially line scanned display device. The circuit is comprised of a threaded refresh buffer connected to receive the data signals and adapted to sort the data signals into groups ordered by extremal line scan positions for the pattern represented. An intermediate buffer has a first input connected to the output of the threaded refresh buffer for storing the ordered data signals once during each display field for the display of the pattern represented and outputting the ordered data signals in synchronism with the line scans of the display. The graphical pattern generator is connected to the output of the intermediate buffer for decoding the ordered data signals outputted therefrom and generating on a first output line components of the pattern represented which lie along the display line to be scanned. A partial raster assembly storage is connected to the first output line from the graphical pattern generator, to store the components of the pattern represented which lie along the display line to be scanned. The graphical pattern generator modifies the decoded ordered data signals to identify the horizontal coordinate for the intersection of the pattern represented with the next display line to be scanned, and outputs the modified data signal over a second output line to a second input line for storage in the intermediate buffer. The graphical pattern generator omits the output of a modified data signal on the second output line when no components of the pattern will intersect succeeding display lines to be scanned in the field.
Directed beam displays, such as the IBM 2250, permit operator designation of data in the refresh buffer without burdening the host processor. This is possible since the data is stored in the refresh buffer in the location designated by the host processor and in the same coded format as received. Since all of one graphic entity is completely displayed before the next is read from the refresh buffer, a light pen detect can uniquely identify a particular item. Conventional graphic digital television (DTV) with its bit per picture element refresh buffer, has lacked this capability.
The on-the-fly digital television display system disclosed in the above referenced Hogan, et al. application, is more amenable to operator designation of data than other known graphic digital television systems, including both bit per element and on-the-fly systems. The invention disclosed herein provides a means for implementing the operator designation function in that system.